With 13,000 advance tickets sold, the first substantial exhibition in Britain of the 17th-century Spanish master Velázquez, which opened yesterday, is certain to be a bigger draw for the National Gallery than any of its previous blockbusters - including Vermeer, Titian and Caravaggio.

But the work that is absorbing devotees of Spain's greatest painter and pre-eminent exponent of baroque is to be found a short distance across London, in the former French embassy building which houses the Wallace Collection. Here resides Velázquez's work Lady with a Fan (c1630-1650), which has been the source of long arguments over the identity of the elegant sitter who betrays the faintest hint of décollete.

The received wisdom is that she is one of the many Spanish courtesans painted by Velázquez during his 43 years as court painter for King Philip IV of Spain.

But a British art historian, Zahira Veliz Bomford, has presented a robust challenge, claiming the subject is an intelligent and feisty French aristocrat, Marie de Rohan-Montbazon, the Duchess of Chevreuse who was forced to flee France on horseback over the Pyreneesdisguised as a man after her volatile clashes with Louis XIII's minister Cardinal Richelieu endangered her life.

Ms Bomford takes as her starting point a letter of 1638 recounting that Velázquez was painting the French aristocrat, "in French air and dress". She also makes the case that the costume of the sitter - white gloves, and daring blue noeud d'amour (love knot) - was out of keeping with the highly conservative, even repressed, Spanish women of the day. The brown silk of her dress is typically French, as is her hairstyle and accessories, Ms Bomford argues. Her tied noeud d'amour was also an extremely popular trinket in France, as was the fan the sitter displays, especially shown in the open "action form" as painted by Velázquez; Spanish women kept theirs closed.

So conservative was Spanish culture at that time that it was not even possible to paint Spanish ladies in the guise of goddesses, a practice popular in northern Europe where, in another intriguing pointer to the provenance of the work, the Duchess of Chevreuse also happened to be painted 11 years earlier by Deruet as Diana in a picture which bears a striking resemblance to the sitter in The Lady With a Fan.

If Ms Bomford is correct, then Velázquez - whose works are known for inviting exploration of the circumstances of their making - was painting one of the most colourful women in mid-17th century Spain, who became a celebrity at Philip's court after reaching Spain in November 1638.

She left France in a hurry after participating in conspiracies against Louis XIII's ministers. Her status as a French enemy of Richelieu put Philip IV, then at war with France, in a difficult position, yet she was welcomed into the court, perhaps as a result of her friendship with the Spanish-born French Queen, Anne of Austria.

The trustees of the Wallace Collection, who may not lend out works under the terms of the original bequest to the nation, are exhibiting the work next to another beautiful female portrait attributed to Velázquez, the Portrait of a Young Lady, from Chatsworth House. Many art historians believe the sitter in both portraits to be the same individual.

At the National Gallery, four rooms have been cleared to make way for the 46 works by "le peintre des peintres," as the French artist Edouard Manet once described Velázquez. These include nine belonging to the Gallery, which has the largest collection outside Spain, and eight from Madrid's Prado.

The exhibition will include one of the jewels in the gallery's collection, The Rokeby Venus - the only surviving example of a female nude by Velázquez, painted at a time when that sort of thing was frowned on.

"Velázquez is perhaps the very greatest painter, a painter who was in tune with his medium - the stuff of paint," said Dawson Carr, the exhibition's curator.

Velázquez - The Lady with a Fan Revealed? is on view at Hertford House, Manchester Square, London W1, until 14 November

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